The origin of the Cameroon Volcanic Line (CVL), which is difficult to explain with traditional plate tectonics and mantle convection models because the volcanism does not display clear age progression, remains widely debated. Existing seismic tomography models show anomalously slow structure beneath the CVL, which some have interpreted to reflect upper mantle convective processes, possibly associated with edge-driven flow related to the neighboring Congo Craton. However, mid- and lower mantle depths are generally not well resolved in these models, making it difficult to determine the extent of the anomalous CVL structure. Here we present a new P-wave velocity model for the African mantle, developed with the largest collection of travel-time residuals recorded across the continent to date and an adaptive model parameterization. Our extensive dataset and inversion method yield high resolution images of the mantle structure beneath western Africa, particularly at the critical mid- and lower mantle depths needed to further evaluate processes associated with the formation of the CVL. Our new model provides strong evidence for a connection between the African Large Low Velocity Province, centered in the lower mantle beneath southern Africa, and the CVL. We suggest that seismically slow material generated near the core-mantle boundary beneath southern Africa moves northwestward under the Congo Craton. At the northern edge of the craton, the hot, buoyant material rises through the upper mantle, causing the CVL volcanism. Consequently, CVL magmatism is driven by large-scale mantle processes rooted in the deep mantle.